High silica glasses are glasses consolidated from a porous state and containing, exclusive of additives, over about 94% silica. Such glasses are also referred to as reconstructed and 96% silica glasses. They are similar to fused silica in their ability to resist softening at high temperatures. Of particular interest, high silica glasses have coefficients of thermal expansion (CTEs) on the order of 7-14.times.10.sup.-7 /.degree.C. This property renders them resistant to heat shock.
Production of such glasses was first disclosed in U.S. Pat. No. 2,215,039 and No. 2,286,275 (Hood et al.). As there described, such glasses are melted as a precursor, phase-separable, borosilicate glass. This glass is then heat treated to separate the glass into a silica-rich matrix phase and a borate-rich second phase. The latter phase is readily soluble, and may be removed by solution in a mineral acid such as nitric acid. The porous glass matrix that remains is largely silica. It may then be thermally consolidated to produce a non-porous, clear glass of lesser volume and having a silica content of at least 94%.
It was subsequently learned that porous, high silica glasses could be impregnated with solutions containing a variety of different metal salts. These salts convert to oxides during consolidation and produce a modified glass. The production of colored glasses in this manner is taught in U.S. Pat. No. 2,303,756, No. 2,340,013 and No. 2,355,746 (Nordberg et al.). Among the colors reported is a yellowish-green obtained with a chromium oxide addition.
Recently, it has become desirable to provide a glass having a controlled yellow color for external automotive lighting. With the advent of halide lamps, glasses used in lamp envelopes have been subjected to greatly increased service temperatures. The ordinary lamp glasses heretofore used may soften at these temperatures. This has led to an interest in high silica glasses for use in such lamps, and more particularly to a high silica glass capable of exhibiting a prescribed yellow color.
Accordingly, it is a basic purpose of the present invention to provide high silica glasses that meet such needs. Another purpose is to provide a reliable method of developing the desired color in a high silica glass.